1. Field of the Invention
The present invention relates to a pattern forming method and a method of fabricating a device, and more particularly, it relates to a pattern forming method of transferring a bright line pattern to a photosensitive material through a photomask having the bright line pattern and a method of fabricating a device.
2. Description of the Prior Art
In recent years, a semiconductor integrated circuit is remarkably highly integrated and refined. Following such integration and refinement of the semiconductor integrated circuit, refinement of a circuit pattern formed on a semiconductor substrate hereinafter simply referred to as a wafer) is also rapidly progressed.
In particular, photolithography is widely recognized as a basic technique in pattern formation. Therefore, various developments and improvements have been made in relation to the photolithography. However, refinement of patterns is increasingly progressed, along with stronger requirement for improvement of pattern resolution.
The photolithography is a technique of transferring a pattern formed on a photomask (subject copy) to a photoresist material applied onto a wafer and patterning a lower etched film with the photoresist material having the transferred pattern.
In this transfer step, the photoresist material is developed. In this development, part irradiated with light is removed in a positive photoresist material, while part not irradiated with light is removed in a negative photoresist material.
In general, the resolution limit R (nm) in photolithography employing reduction exposure is expressed as follows:
R=k1xc2x7xcex/(NA)
where xcex represents the wavelength (nm) of employed light, NA represents the numerical aperture of a projection optical system of a lens, and k1 represents a constant depending on imaging conditions and a resist process.
As understood from the above equation, the constant k1 and the wavelength xcex may be reduced while increasing the numerical aperture NA, in order to improve the resolution limit R, i.e., in order to obtain a fine pattern. In other words, the constant k1 depending on the resist process may be reduced while the wavelength xcex may be reduced and the numerical aperture NA may be increased.
In this case, it is technically difficult to reduce the wavelength of a light source, and the numerical aperture NA must be increased at the same wavelength. If the numerical aperture NA is increased, however, the depth xcex4 of focus (xcex4=k2xc2x7xcex/(NA)2) is reduced to disadvantageously deteriorate the shape and the dimensional accuracy of the formed pattern.
In general, a fine pattern formed on a fine bright line mask employed as a photomask is transferred to a photoresist material by performing exposure with a light exposure of 2.5 to three times the light exposure (Eth) inverting solubility of the photoresist material. The exposure is performed with such a light exposure for substantially equalizing the pattern dimension in the photomask and the finished dimension of the pattern transferred onto the photoresist material to each other and simplifying the pattern design.
In order to transfer the pattern of the photomask to the photoresist material by exposure with high resolution in such photolithography, the photoresist material must be conformed with the best focal plane of the projection optical system within the range of the depth of focus.
When exposure is performed under the aforementioned condition, however, the CD (critical dimension) is so remarkably fluctuated by focal change that excellent CD-focus characteristics cannot be obtained. In other words, the dimension of the pattern formed on the photoresist material is remarkably fluctuated following focal change. In transfer onto a finite step on an actual substrate, therefore, the focus changes by about this step and hence the critical dimension is disadvantageously remarkably dispersed in correspondence thereto.
An object of the present invention is to provide a pattern forming method and a method of fabricating a device increasing a depth of focus by attaining excellent CD-focus characteristics thereby reducing dispersion of a critical dimension.
The pattern forming method according to the present invention, employing a photomask having a bright line pattern for transferring the bright line pattern to a photosensitive material, comprises a step of transferring the bright line pattern of the photomask to the photosensitive material by exposure with a light exposure of at least four times and not more than 10 times the light exposure (Eth) for photosensitizing the photosensitive material and inverting solubility.
The inventor has made deep study in order to solve the aforementioned problem, to find that light intensity keeping the pattern dimension of a photoresist material substantially unchanged following focal change is present within a light exposure range of at least four times and not more than 10 times the light exposure (Eth) for photosensitizing the photoresist material and inverting solubility.
According to the inventive pattern forming method, exposure is performed with light intensity based on this recognition, whereby iso-focal characteristics are obtained for keeping the pattern dimension of the photoresist material substantially unchanged following focal change, so that remarkably excellent CD-focal characteristics can be obtained and the depth of focus (DOF) can be increased.
When exposure is performed with the light exposure of at least four times and not more than 10 times the light exposure (Eth), the finished pattern dimension of the photoresist material differs from the pattern dimension on the photomask. Also in this case, however, design of a device pattern is not complicated due to the recent development of the CAD (computer-aided design) technique.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.